The highway department uses reflective green or blue backgrounds on big signs, with reflective white lettering. Typically on signs like one way signs or other smaller traffic signs, the lettering is screenprinted onto a white reflective background. The lettering itself isn't really very reflective in that case.
True, Interstate Highway signs are reflective on reflective. You will also note that the letter spacing on these signs is wide for increased legibility. This is to counter the effect of "halation." A letter that is lighted, or that glows, increases in size visually. The light glow tends to blur the letter edges and fill in the spaces between letters, so increased letter spacing helps counter the effect. This is why chunky channel letters, spaced tightly, are often difficult to read.
In addition, another phenomenon, "irradiation," can make light letters on a dark background appear larger and thicker even when not reflective.
Why white looks bigger than black —the Irradiation Illusion
Reflective signs that are screen printed are often printed with an ink that is somewhat transparent to allow the reflectivity of the background to show through. This is how Grimco, a wholesaler in St. Louis, typically creates their reflective street signs. But, as mentioned above by unclebun, the reflectivity through the printed graphics is not that great. I've also noticed that some of these colors screened onto reflective vinyl fade fairly quickly.
The various stock colors of reflective vinyl have varying degrees of light reflection, too. I don't have my chart in front of me, but it seems like black reflects only 10% of the light, compared to 90% for white, with all the other colors ranged in between.
Brad in Kansas City